marine creature vs beast

marine creature and beast both are nouns.

Word	Noun	Adjective	Verb	Adverb
marine creature	Yes	No	No	No
beast	Yes	No	No	No

As nouns, beast is a hypernym of marine creature; that is, beast is a word with a broader meaning than marine creature:

marine creature: any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals
beast: a living organism characterized by voluntary movement

Other hypernyms of marine creature include animal, animate being, brute, creature, fauna.

marine creature (noun)	beast (noun)
any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals	a living organism characterized by voluntary movement
	a cruelly rapacious person